i) Field of the Invention
The present invention relates to a telephone capable of inhibiting the sending of a particular selection signal to control an expensive long distance call.
ii) Description of the Related Arts
FIG. 4 illustrates a conventional telephone. In this telephone, a line switch 1 for connecting or disconnecting lines L1 and L2 is provided between a telephone exchange 2 and the telephone and executes a connecting or disconnection operation in response to an internal hook signal output from a CPU 8. One end of the line switch 1 is coupled with a DP (dial pulse) switching circuit 4 via a DB (diode bridge) circuit 3 and the DP switching circuit 4 is connected to a speech network circuit 5 to which a handset 6 is coupled.
A hook switch 7 which is operated in response to a mount or demount of the handset 6, and a key board 9 for inputting a dial number are linked to the CPU 8. The CPU 8 outputs the internal hook signal according to the input of the hook signal output from the hook switch 7 and carries out a coding of the key input from the key board 9 to output code data. Also, the CPU 8 is coupled with a first digit zero dial control circuit 10 and the first digit zero dial control circuit 10 is connected to a selection signal out circuit 11. The selection signal out circuit 11 outputs a DP signal to the DP switching circuit 4 or a tone signal such as a DTMF (dual tone multi-frequency obtained by combining two frequencies in an audio frequency band) signal to the speech network circuit 5.
Next, the operation of the above-described conventional telephone shown in FIG. 4 will now described.
First, when the handset 6 is lifted, the hook switch 7 is switched to a broken line position in response to the lift operation of the handset 6 and the a low level hook signal "L" is input to the CPU 8. Then, based on the low level hook signal, the CPU 8 outputs the internal hook signal so as to switch the line switch 1 to a broken line position. As a result, lines L1 and L2 form a loop.
In this state, when a dial number of a destination party or person is input from the key board 9, the CPU 8 codes the key input and outputs the code data to the first digit zero dial control circuit 10. When the first digit of this code data sent from the CPU 8 is not "0", this code data pass unchanged through the first digit zero dial control circuit 10 and is input to the selection signal out circuit 11. Hence, the selection signal out circuit 11 outputs the DP signal or the DTMF signal to the DP switching circuit 4 or the speech network circuit 5 on the basis of the code data. As a result, the telephone is linked to the destination party via the lines L1 and L2 and the telephone station exchange 2.
However, in case of a dialed number being of the destination party located outside a city (long distance), the first digit of this dial number input from the key board 9 is "0" (or "1" in the United States) and a code corresponding to this zero is input to the CPU 8. The CPU 8 discriminates that the first digit of the code is "0" and the code of this dialed number is not input to the selection signal out circuit 11. Thus the telephone is not linked with the destination party. Accordingly, the placing of an expensive long distance call or international call can be exactly inhibited.
Since the conventional telephone is constructed as described above, the placing of an expensive call such as the long distance call or the international call can be inhibited. However, when using a DTMF generator 12, such as a remote commander or the like, which is capable of recording the dialed number of the destination party as a DTMF signal and outputting the sound of this dialed number, the aforementioned transmission rule is invalidated. That is, when the DTMF signal of the remote commander is input to the speech network circuit 5 through a transmitter of the handset 6 and not through the CPU 8, the DTMF signal is sent out to the telephone exchange 2 via the DP switching circuit 4, the DB circuit 3, the line switch 1 and the lines L1 and L2 in the same manner as the case of the DTMF signal of the selection signal out circuit 11, and the telephone is linked to the destination party. Hence, the expensive call such as the international call to be inhibited can be carried out and the transmission rule is insufficient.